package com.autumn.util;/*
						* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
						* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
						*/

import java.util.*;
import java.util.function.Consumer;

/**
 * Doubly-linked list implementation of the {@code List} and {@code Deque}
 * interfaces. Implements all optional list operations, and permits all elements
 * (including {@code null}).
 *
 * <p>
 * All of the operations perform as could be expected for a doubly-linked list.
 * Operations that index into the list will traverse the list from the beginning
 * or the end, whichever is closer to the specified index.
 *
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If
 * multiple threads access a linked list concurrently, and at least one of the
 * threads modifies the list structurally, it <i>must</i> be synchronized
 * externally. (A structural modification is any operation that adds or deletes
 * one or more elements; merely setting the value of an element is not a
 * structural modification.) This is typically accomplished by synchronizing on
 * some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList} method.
 * This is best done at creation time, to prevent accidental unsynchronized
 * access to the list:
 * 
 * <pre>
 *   List list = Collections.synchronizedList(new DLinkedList(...));
 * </pre>
 *
 * <p>
 * The iterators returned by this class's {@code iterator} and
 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in any way
 * except through the Iterator's own {@code remove} or {@code add} methods, the
 * iterator will throw a {@link ConcurrentModificationException}. Thus, in the
 * face of concurrent modification, the iterator fails quickly and cleanly,
 * rather than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * <p>
 * Note that the fail-fast behavior of an iterator cannot be guaranteed as it
 * is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification. Fail-fast iterators throw
 * {@code ConcurrentModificationException} on a best-effort basis. Therefore, it
 * would be wrong to write a program that depended on this exception for its
 * correctness: <i>the fail-fast behavior of iterators should be used only to
 * detect bugs.</i>
 *
 * <p>
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> Java
 * Collections Framework</a>.
 *
 * @author Josh Bloch
 * @see List
 * @see ArrayList
 * @since 1.2
 * @param <E>
 *            the type of elements held in this collection
 */

public class DLinkedList<E> extends AbstractSequentialList<E>
		implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
	transient int size = 0;

	/**
	 * Pointer to first node. Invariant: (first == null && last == null) ||
	 * (first.prev == null && first.item != null)
	 */
	transient Node<E> first;

	/**
	 * Pointer to last node. Invariant: (first == null && last == null) ||
	 * (last.next == null && last.item != null)
	 */
	transient Node<E> last;

	/**
	 * Constructs an empty list.
	 */
	public DLinkedList() {
	}

	/**
	 * Constructs a list containing the elements of the specified collection, in the
	 * order they are returned by the collection's iterator.
	 *
	 * @param c
	 *            the collection whose elements are to be placed into this list
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public DLinkedList(Collection<? extends E> c) {
		this();
		addAll(c);
	}

	/**
	 * Links e as first element.
	 */
	private void linkFirst(E e) {
		final Node<E> f = first;
		final Node<E> newNode = new Node<>(null, e, f);
		first = newNode;
		if (f == null) {
			last = newNode;
		} else {
			f.prev = newNode;
		}
		size++;
		modCount++;
	}

	/**
	 * Links e as last element.
	 */
	void linkLast(E e) {
		final Node<E> l = last;
		final Node<E> newNode = new Node<>(l, e, null);
		last = newNode;
		if (l == null) {
			first = newNode;
		} else {
			l.next = newNode;
		}
		size++;
		modCount++;
	}

	/**
	 * Inserts element e before non-null Node succ.
	 */
	void linkBefore(E e, Node<E> succ) {
		// assert succ != null;
		final Node<E> pred = succ.prev;
		final Node<E> newNode = new Node<>(pred, e, succ);
		succ.prev = newNode;
		if (pred == null)
			first = newNode;
		else
			pred.next = newNode;
		size++;
		modCount++;
	}

	/**
	 * Unlinks non-null first node f.
	 */
	private E unlinkFirst(Node<E> f) {
		// assert f == first && f != null;
		final E element = f.item;
		final Node<E> next = f.next;
		f.item = null;
		f.next = null; // help GC
		first = next;
		if (next == null)
			last = null;
		else
			next.prev = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Unlinks non-null last node l.
	 */
	private E unlinkLast(Node<E> l) {
		// assert l == last && l != null;
		final E element = l.item;
		final Node<E> prev = l.prev;
		l.item = null;
		l.prev = null; // help GC
		last = prev;
		if (prev == null)
			first = null;
		else
			prev.next = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Unlinks non-null node x.
	 */
	E unlink(Node<E> x) {
		// assert x != null;
		final E element = x.item;
		final Node<E> next = x.next;
		final Node<E> prev = x.prev;

		if (prev == null) {
			first = next;
		} else {
			prev.next = next;
			x.prev = null;
		}

		if (next == null) {
			last = prev;
		} else {
			next.prev = prev;
			x.next = null;
		}

		x.item = null;
		size--;
		modCount++;
		return element;
	}

	/**
	 * Returns the first element in this list.
	 *
	 * @return the first element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E getFirst() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return f.item;
	}

	/**
	 * Returns the first node in this list.
	 *
	 * @return the first node in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public Node<E> getFirstNode() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return f;
	}


	/**
	 * Returns the last element in this list.
	 *
	 * @return the last element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E getLast() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return l.item;
	}


	/**
	 * Returns the last node in this list.
	 *
	 * @return the last node in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public Node<E> getLastNode() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return l;
	}


	/**
	 * Removes and returns the first element from this list.
	 *
	 * @return the first element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E removeFirst() {
		final Node<E> f = first;
		if (f == null)
			throw new NoSuchElementException();
		return unlinkFirst(f);
	}

	/**
	 * Removes and returns the last element from this list.
	 *
	 * @return the last element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	public E removeLast() {
		final Node<E> l = last;
		if (l == null)
			throw new NoSuchElementException();
		return unlinkLast(l);
	}

	/**
	 * Inserts the specified element at the beginning of this list.
	 *
	 * @param e
	 *            the element to add
	 */
	public void addFirst(E e) {
		linkFirst(e);
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #add}.
	 *
	 * @param e
	 *            the element to add
	 */
	public void addLast(E e) {
		linkLast(e);
	}

	/**
	 * Returns {@code true} if this list contains the specified element. More
	 * formally, returns {@code true} if and only if this list contains at least one
	 * element {@code e} such that
	 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
	 *
	 * @param o
	 *            element whose presence in this list is to be tested
	 * @return {@code true} if this list contains the specified element
	 */
	public boolean contains(Object o) {
		return indexOf(o) != -1;
	}

	/**
	 * Returns the number of elements in this list.
	 *
	 * @return the number of elements in this list
	 */
	public int size() {
		return size;
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addLast}.
	 *
	 * @param e
	 *            element to be appended to this list
	 * @return {@code true} (as specified by {@link Collection#add})
	 */
	public boolean add(E e) {
		linkLast(e);
		return true;
	}

	/**
	 * Removes the first occurrence of the specified element from this list, if it
	 * is present. If this list does not contain the element, it is unchanged. More
	 * formally, removes the element with the lowest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt> (if
	 * such an element exists). Returns {@code true} if this list contained the
	 * specified element (or equivalently, if this list changed as a result of the
	 * call).
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if this list contained the specified element
	 */
	public boolean remove(Object o) {
		if (o == null) {
			for (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Appends all of the elements in the specified collection to the end of this
	 * list, in the order that they are returned by the specified collection's
	 * iterator. The behavior of this operation is undefined if the specified
	 * collection is modified while the operation is in progress. (Note that this
	 * will occur if the specified collection is this list, and it's nonempty.)
	 *
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public boolean addAll(Collection<? extends E> c) {
		return addAll(size, c);
	}

	/**
	 * Inserts all of the elements in the specified collection into this list,
	 * starting at the specified position. Shifts the element currently at that
	 * position (if any) and any subsequent elements to the right (increases their
	 * indices). The new elements will appear in the list in the order that they are
	 * returned by the specified collection's iterator.
	 *
	 * @param index
	 *            index at which to insert the first element from the specified
	 *            collection
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public boolean addAll(int index, Collection<? extends E> c) {
		checkPositionIndex(index);

		Object[] a = c.toArray();
		int numNew = a.length;
		if (numNew == 0)
			return false;

		Node<E> pred, succ;
		if (index == size) {
			succ = null;
			pred = last;
		} else {
			succ = node(index);
			pred = succ.prev;
		}

		for (Object o : a) {
			@SuppressWarnings("unchecked")
			E e = (E) o;
			Node<E> newNode = new Node<>(pred, e, null);
			if (pred == null)
				first = newNode;
			else
				pred.next = newNode;
			pred = newNode;
		}

		if (succ == null) {
			last = pred;
		} else {
			pred.next = succ;
			succ.prev = pred;
		}

		size += numNew;
		modCount++;
		return true;
	}

	/**
	 * Removes all of the elements from this list. The list will be empty after this
	 * call returns.
	 */
	public void clear() {
		// Clearing all of the links between nodes is "unnecessary", but:
		// - helps a generational GC if the discarded nodes inhabit
		// more than one generation
		// - is sure to free memory even if there is a reachable Iterator
		for (Node<E> x = first; x != null;) {
			Node<E> next = x.next;
			x.item = null;
			x.next = null;
			x.prev = null;
			x = next;
		}
		first = last = null;
		size = 0;
		modCount++;
	}

	// Positional Access Operations

	/**
	 * Returns the element at the specified position in this list.
	 *
	 * @param index
	 *            index of the element to return
	 * @return the element at the specified position in this list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E get(int index) {
		checkElementIndex(index);
		return node(index).item;
	}

	/**
	 * Replaces the element at the specified position in this list with the
	 * specified element.
	 *
	 * @param index
	 *            index of the element to replace
	 * @param element
	 *            element to be stored at the specified position
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E set(int index, E element) {
		checkElementIndex(index);
		Node<E> x = node(index);
		E oldVal = x.item;
		x.item = element;
		return oldVal;
	}

	/**
	 * Inserts the specified element at the specified position in this list. Shifts
	 * the element currently at that position (if any) and any subsequent elements
	 * to the right (adds one to their indices).
	 *
	 * @param index
	 *            index at which the specified element is to be inserted
	 * @param element
	 *            element to be inserted
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public void add(int index, E element) {
		checkPositionIndex(index);

		if (index == size)
			linkLast(element);
		else
			linkBefore(element, node(index));
	}

	/**
	 * Removes the element at the specified position in this list. Shifts any
	 * subsequent elements to the left (subtracts one from their indices). Returns
	 * the element that was removed from the list.
	 *
	 * @param index
	 *            the index of the element to be removed
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	public E remove(int index) {
		checkElementIndex(index);
		return unlink(node(index));
	}

	/**
	 * Tells if the argument is the index of an existing element.
	 */
	private boolean isElementIndex(int index) {
		return index >= 0 && index < size;
	}

	/**
	 * Tells if the argument is the index of a valid position for an iterator or an
	 * add operation.
	 */
	private boolean isPositionIndex(int index) {
		return index >= 0 && index <= size;
	}

	/**
	 * Constructs an IndexOutOfBoundsException detail message. Of the many possible
	 * refactorings of the error handling code, this "outlining" performs best with
	 * both server and client VMs.
	 */
	private String outOfBoundsMsg(int index) {
		return "Index: " + index + ", Size: " + size;
	}

	private void checkElementIndex(int index) {
		if (!isElementIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}

	private void checkPositionIndex(int index) {
		if (!isPositionIndex(index))
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
	}

	/**
	 * Returns the (non-null) Node at the specified element index.
	 */
	Node<E> node(int index) {
		// assert isElementIndex(index);

		if (index < (size >> 1)) {
			Node<E> x = first;
			for (int i = 0; i < index; i++)
				x = x.next;
			return x;
		} else {
			Node<E> x = last;
			for (int i = size - 1; i > index; i--)
				x = x.prev;
			return x;
		}
	}

	// Search Operations

	/**
	 * Returns the index of the first occurrence of the specified element in this
	 * list, or -1 if this list does not contain the element. More formally, returns
	 * the lowest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>, or
	 * -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the first occurrence of the specified element in this
	 *         list, or -1 if this list does not contain the element
	 */
	public int indexOf(Object o) {
		int index = 0;
		if (o == null) {
			for (Node<E> x = first; x != null; x = x.next) {
				if (x.item == null)
					return index;
				index++;
			}
		} else {
			for (Node<E> x = first; x != null; x = x.next) {
				if (o.equals(x.item))
					return index;
				index++;
			}
		}
		return -1;
	}

	/**
	 * Returns the index of the last occurrence of the specified element in this
	 * list, or -1 if this list does not contain the element. More formally, returns
	 * the highest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>, or
	 * -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the last occurrence of the specified element in this
	 *         list, or -1 if this list does not contain the element
	 */
	public int lastIndexOf(Object o) {
		int index = size;
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (x.item == null)
					return index;
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				index--;
				if (o.equals(x.item))
					return index;
			}
		}
		return -1;
	}

	// Queue operations.

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	public E peek() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	public E element() {
		return getFirst();
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	public E poll() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	public E remove() {
		return removeFirst();
	}

	/**
	 * Adds the specified element as the tail (last element) of this list.
	 *
	 * @param e
	 *            the element to add
	 * @return {@code true} (as specified by {@link Queue#offer})
	 * @since 1.5
	 */
	public boolean offer(E e) {
		return add(e);
	}

	// Deque operations
	/**
	 * Inserts the specified element at the front of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerFirst})
	 * @since 1.6
	 */
	public boolean offerFirst(E e) {
		addFirst(e);
		return true;
	}

	/**
	 * Inserts the specified element at the end of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerLast})
	 * @since 1.6
	 */
	public boolean offerLast(E e) {
		addLast(e);
		return true;
	}

	/**
	 * Retrieves, but does not remove, the first element of this list, or returns
	 * {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E peekFirst() {
		final Node<E> f = first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the last element of this list, or returns
	 * {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E peekLast() {
		final Node<E> l = last;
		return (l == null) ? null : l.item;
	}

	/**
	 * Retrieves and removes the first element of this list, or returns {@code null}
	 * if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E pollFirst() {
		final Node<E> f = first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the last element of this list, or returns {@code null}
	 * if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	public E pollLast() {
		final Node<E> l = last;
		return (l == null) ? null : unlinkLast(l);
	}

	/**
	 * Pushes an element onto the stack represented by this list. In other words,
	 * inserts the element at the front of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addFirst}.
	 *
	 * @param e
	 *            the element to push
	 * @since 1.6
	 */
	public void push(E e) {
		addFirst(e);
	}

	/**
	 * Pops an element from the stack represented by this list. In other words,
	 * removes and returns the first element of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #removeFirst()}.
	 *
	 * @return the element at the front of this list (which is the top of the stack
	 *         represented by this list)
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.6
	 */
	public E pop() {
		return removeFirst();
	}

	/**
	 * Removes the first occurrence of the specified element in this list (when
	 * traversing the list from head to tail). If the list does not contain the
	 * element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	public boolean removeFirstOccurrence(Object o) {
		return remove(o);
	}

	/**
	 * Removes the last occurrence of the specified element in this list (when
	 * traversing the list from head to tail). If the list does not contain the
	 * element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	public boolean removeLastOccurrence(Object o) {
		if (o == null) {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = last; x != null; x = x.prev) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Returns a list-iterator of the elements in this list (in proper sequence),
	 * starting at the specified position in the list. Obeys the general contract of
	 * {@code List.listIterator(int)}.
	 * <p>
	 *
	 * The list-iterator is <i>fail-fast</i>: if the list is structurally modified
	 * at any time after the Iterator is created, in any way except through the
	 * list-iterator's own {@code remove} or {@code add} methods, the list-iterator
	 * will throw a {@code ConcurrentModificationException}. Thus, in the face of
	 * concurrent modification, the iterator fails quickly and cleanly, rather than
	 * risking arbitrary, non-deterministic behavior at an undetermined time in the
	 * future.
	 *
	 * @param index
	 *            index of the first element to be returned from the list-iterator
	 *            (by a call to {@code next})
	 * @return a ListIterator of the elements in this list (in proper sequence),
	 *         starting at the specified position in the list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @see List#listIterator(int)
	 */
	public ListIterator<E> listIterator(int index) {
		checkPositionIndex(index);
		return new ListItr(index);
	}

	private class ListItr implements ListIterator<E> {
		private Node<E> lastReturned;
		private Node<E> next;
		private int nextIndex;
		private int expectedModCount = modCount;

		ListItr(int index) {
			// assert isPositionIndex(index);
			next = (index == size) ? null : node(index);
			nextIndex = index;
		}

		public boolean hasNext() {
			return nextIndex < size;
		}

		public E next() {
			checkForComodification();
			if (!hasNext())
				throw new NoSuchElementException();

			lastReturned = next;
			next = next.next;
			nextIndex++;
			return lastReturned.item;
		}

		public boolean hasPrevious() {
			return nextIndex > 0;
		}

		public E previous() {
			checkForComodification();
			if (!hasPrevious())
				throw new NoSuchElementException();

			lastReturned = next = (next == null) ? last : next.prev;
			nextIndex--;
			return lastReturned.item;
		}

		public int nextIndex() {
			return nextIndex;
		}

		public int previousIndex() {
			return nextIndex - 1;
		}

		public void remove() {
			checkForComodification();
			if (lastReturned == null)
				throw new IllegalStateException();

			Node<E> lastNext = lastReturned.next;
			unlink(lastReturned);
			if (next == lastReturned)
				next = lastNext;
			else
				nextIndex--;
			lastReturned = null;
			expectedModCount++;
		}

		public void set(E e) {
			if (lastReturned == null)
				throw new IllegalStateException();
			checkForComodification();
			lastReturned.item = e;
		}

		public void add(E e) {
			checkForComodification();
			lastReturned = null;
			if (next == null)
				linkLast(e);
			else
				linkBefore(e, next);
			nextIndex++;
			expectedModCount++;
		}

		public void forEachRemaining(Consumer<? super E> action) {
			Objects.requireNonNull(action);
			while (modCount == expectedModCount && nextIndex < size) {
				action.accept(next.item);
				lastReturned = next;
				next = next.next;
				nextIndex++;
			}
			checkForComodification();
		}

		final void checkForComodification() {
			if (modCount != expectedModCount)
				throw new ConcurrentModificationException();
		}
	}

	/**
	 * 节点
	 * 
	 * @author 老码农 2018-03-25 00:26:31
	 * @param <E>
	 */
	public static class Node<E> {
		private E item;
		private Node<E> next;
		private Node<E> prev;

		Node(Node<E> prev, E element, Node<E> next) {
			this.item = element;
			this.next = next;
			this.prev = prev;
		}

		public E getItem() {
			return item;
		}

		public Node<E> getNext() {
			return next;
		}

		public Node<E> getPrev() {
			return prev;
		}

	}

	/**
	 * @since 1.6
	 */
	public Iterator<E> descendingIterator() {
		return new DescendingIterator();
	}

	/**
	 * Adapter to provide descending iterators via ListItr.previous
	 */
	private class DescendingIterator implements Iterator<E> {
		private final ListItr itr = new ListItr(size());

		public boolean hasNext() {
			return itr.hasPrevious();
		}

		public E next() {
			return itr.previous();
		}

		public void remove() {
			itr.remove();
		}
	}

	@SuppressWarnings("unchecked")
	private DLinkedList<E> superClone() {
		try {
			return (DLinkedList<E>) super.clone();
		} catch (CloneNotSupportedException e) {
			throw new InternalError(e);
		}
	}

	/**
	 * Returns a shallow copy of this {@code DLinkedList}. (The elements themselves
	 * are not cloned.)
	 *
	 * @return a shallow copy of this {@code DLinkedList} instance
	 */
	public Object clone() {
		DLinkedList<E> clone = superClone();

		// Put clone into "virgin" state
		clone.first = clone.last = null;
		clone.size = 0;
		clone.modCount = 0;

		// Initialize clone with our elements
		for (Node<E> x = first; x != null; x = x.next)
			clone.add(x.item);

		return clone;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper
	 * sequence (from first to last element).
	 *
	 * <p>
	 * The returned array will be "safe" in that no references to it are maintained
	 * by this list. (In other words, this method must allocate a new array). The
	 * caller is thus free to modify the returned array.
	 *
	 * <p>
	 * This method acts as bridge between array-based and collection-based APIs.
	 *
	 * @return an array containing all of the elements in this list in proper
	 *         sequence
	 */
	public Object[] toArray() {
		Object[] result = new Object[size];
		int i = 0;
		for (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;
		return result;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper
	 * sequence (from first to last element); the runtime type of the returned array
	 * is that of the specified array. If the list fits in the specified array, it
	 * is returned therein. Otherwise, a new array is allocated with the runtime
	 * type of the specified array and the size of this list.
	 *
	 * <p>
	 * If the list fits in the specified array with room to spare (i.e., the array
	 * has more elements than the list), the element in the array immediately
	 * following the end of the list is set to {@code null}. (This is useful in
	 * determining the length of the list <i>only</i> if the caller knows that the
	 * list does not contain any null elements.)
	 *
	 * <p>
	 * Like the {@link #toArray()} method, this method acts as bridge between
	 * array-based and collection-based APIs. Further, this method allows precise
	 * control over the runtime type of the output array, and may, under certain
	 * circumstances, be used to save allocation costs.
	 *
	 * <p>
	 * Suppose {@code x} is a list known to contain only strings. The following code
	 * can be used to dump the list into a newly allocated array of {@code STRING}:
	 *
	 * <pre>
	 * STRING[] y = x.toArray(new STRING[0]);
	 * </pre>
	 *
	 * Note that {@code toArray(new Object[0])} is identical in function to
	 * {@code toArray()}.
	 *
	 * @param a
	 *            the array into which the elements of the list are to be stored, if
	 *            it is big enough; otherwise, a new array of the same runtime type
	 *            is allocated for this purpose.
	 * @return an array containing the elements of the list
	 * @throws ArrayStoreException
	 *             if the runtime type of the specified array is not a supertype of
	 *             the runtime type of every element in this list
	 * @throws NullPointerException
	 *             if the specified array is null
	 */
	@SuppressWarnings("unchecked")
	public <T> T[] toArray(T[] a) {
		if (a.length < size)
			a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size);
		int i = 0;
		Object[] result = a;
		for (Node<E> x = first; x != null; x = x.next)
			result[i++] = x.item;

		if (a.length > size)
			a[size] = null;

		return a;
	}

	private static final long serialVersionUID = 876323262645176354L;

	/**
	 * Saves the state of this {@code DLinkedList} instance to a stream (that is,
	 * serializes it).
	 *
	 * @serialData The size of the list (the number of elements it contains) is
	 *             emitted (int), followed by all of its elements (each an Object)
	 *             in the proper order.
	 */
	private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
		// Write out any hidden serialization magic
		s.defaultWriteObject();

		// Write out size
		s.writeInt(size);

		// Write out all elements in the proper order.
		for (Node<E> x = first; x != null; x = x.next)
			s.writeObject(x.item);
	}

	/**
	 * Reconstitutes this {@code DLinkedList} instance from a stream (that is,
	 * deserializes it).
	 */
	@SuppressWarnings("unchecked")
	private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
		// read in any hidden serialization magic
		s.defaultReadObject();

		// read in size
		int size = s.readInt();

		// read in all elements in the proper order.
		for (int i = 0; i < size; i++)
			linkLast((E) s.readObject());
	}

	/**
	 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> and
	 * <em>fail-fast</em> {@link Spliterator} over the elements in this list.
	 *
	 * <p>
	 * The {@code Spliterator} reports {@link Spliterator#SIZED} and
	 * {@link Spliterator#ORDERED}. Overriding implementations should document the
	 * reporting of additional characteristic values.
	 *
	 * @implNote The {@code Spliterator} additionally reports
	 *           {@link Spliterator#SUBSIZED} and implements {@code trySplit} to
	 *           permit limited parallelism..
	 *
	 * @return a {@code Spliterator} over the elements in this list
	 * @since 1.8
	 */
	@Override
	public Spliterator<E> spliterator() {
		return new LLSpliterator<E>(this, -1, 0);
	}

	/** A customized variant of Spliterators.IteratorSpliterator */
	static final class LLSpliterator<E> implements Spliterator<E> {
		static final int BATCH_UNIT = 1 << 10; // batch array size increment
		static final int MAX_BATCH = 1 << 25; // max batch array size;
		final DLinkedList<E> list; // null OK unless traversed
		Node<E> current; // current node; null until initialized
		int est; // size estimate; -1 until first needed
		int expectedModCount; // initialized when est set
		int batch; // batch size for splits

		LLSpliterator(DLinkedList<E> list, int est, int expectedModCount) {
			this.list = list;
			this.est = est;
			this.expectedModCount = expectedModCount;
		}

		final int getEst() {
			int s; // force initialization
			final DLinkedList<E> lst;
			if ((s = est) < 0) {
				if ((lst = list) == null)
					s = est = 0;
				else {
					expectedModCount = lst.modCount;
					current = lst.first;
					s = est = lst.size;
				}
			}
			return s;
		}

		public long estimateSize() {
			return (long) getEst();
		}

		public Spliterator<E> trySplit() {
			Node<E> p;
			int s = getEst();
			if (s > 1 && (p = current) != null) {
				int n = batch + BATCH_UNIT;
				if (n > s)
					n = s;
				if (n > MAX_BATCH)
					n = MAX_BATCH;
				Object[] a = new Object[n];
				int j = 0;
				do {
					a[j++] = p.item;
				} while ((p = p.next) != null && j < n);
				current = p;
				batch = j;
				est = s - j;
				return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
			}
			return null;
		}

		public void forEachRemaining(Consumer<? super E> action) {
			Node<E> p;
			int n;
			if (action == null)
				throw new NullPointerException();
			if ((n = getEst()) > 0 && (p = current) != null) {
				current = null;
				est = 0;
				do {
					E e = p.item;
					p = p.next;
					action.accept(e);
				} while (p != null && --n > 0);
			}
			if (list.modCount != expectedModCount)
				throw new ConcurrentModificationException();
		}

		public boolean tryAdvance(Consumer<? super E> action) {
			Node<E> p;
			if (action == null)
				throw new NullPointerException();
			if (getEst() > 0 && (p = current) != null) {
				--est;
				E e = p.item;
				current = p.next;
				action.accept(e);
				if (list.modCount != expectedModCount)
					throw new ConcurrentModificationException();
				return true;
			}
			return false;
		}

		public int characteristics() {
			return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
		}
	}

}
